1) Skull cuts are pretty, not particularly profitable, particularly not profitable if you sink in 4 large heart cuts to make it, it's much better if you use cheap cuts and small gems.
2) The 4 different gems part was unnecessary, and also dangerous. When you put 5 different cuts you will get a skull cut gem, regardless of what gems or how many different ones you used. The resulting type of gem is chosen at random from what you put in, so you ran a chance of getting a skull cut emerald.
3) The formula produces a large skull cut gem, regardless of the size of what you put in. Using large gems for it is not particularly efficient.